1. Field of the Invention
The present invention relates to video tape control time code reading method and apparatus. Embodiments of the invention may be incorporated into video tape recorders (VTRs) to enable dual-standard VTRs to be made. Such a dual-standard VTR is capable of distinguishing recorded video signals having a frame frequency of 30 frames per second, such as in the National Television System Committee (NTSC) system, and recorded video signals having a frame frequency of 25 frames per second, such as in the International Radio Consultative Committee (CCIR) system.
2. Description of the Prior Art
Most particularly for professional purposes, that is to say for the purposes of people concerned with the production and transmission of television, the editing of video tape is of substantial importance. While such editing can be done by cutting and splicing the video tape, this method is extremely tedious and time-consuming, involves considerable risk of irreparably damaging the video tape, and in any case substantially degrades the quality of signal reproduction which can be obtained from the edited video tape. So-called electronic editing is therefore now preferred, and in this method signals are selectively recorded on a single video tape from more than one source, or an existing signal on a video tape is edited by erasing parts of the recorded signal and recording substitute parts derived from another source. This can be done without introducing any discontinuity into the edited recorded signal subsequently to be reproduced. A critical factor in such editing is the ability to rapidly locate specific parts of a signal, most particularly individual frames. For this purpose the Society of Motion Picture and Television Engineers (SMPTE) in the United States of America and the European Broadcasting Union (EBU) in Europe have proposed respective so-called control time codes for recording on a video tape, for example, on the number 2 audio track or cue track. The SMPTE control time code will be described in detail below, and in fact the EBU control time code is very similar, most of the differences not being material to the present invention. The essential feature of each of the control time codes is that it comprises in binary coded form a time, which may be the time of day, expressed in hours, minutes and seconds, and, individual frame numbers of the recorded video signal within each second. In other words, each frame of the recorded video signal is uniquely identified. The different frame frequencies result in a different bit frequency in the control time code, but the difference is only in the ratio 6:5, and is thus difficult to detect. Moreover, the difference can only be detected when the video tape is being transported at a known speed, which is not the case in fast forward or rewind operation.
These control time codes and VTRs able to read and be controlled making use of these control time codes enormously facilitate editing, but there is a problem, at least for professional users, that different video tapes bearing recorded signals corresponding to different systems, and in particular systems operating at different frame frequencies, need to be distinguishable to ensure appropriate decoding and correct reproduction.